Frequency domain or Fourier domain optical coherence tomography (FD-OCT) is an advanced tomography technology widely used in the medical diagnostic field. In particular, because the FD-OCT technology makes it possible to safely obtain tomographic images of retina and others, it is popular in the ophthalmology field, and is also used for coronary artery diagnosis or for endoscopy.
In addition to the medical field, the FD-OCT technology has also been used in various industrial fields, for example, measuring a thickness of a wafer and analyzing each layer of a picture to preserve the artwork. Optical-fiber-based optical coherence tomography is particularly useful not only for medial endoscopy but also for an industrial use, because it can be used in a hard-to-reach area such as radioactive contaminated environments.
The optical coherence tomography has a superior resolution compared to conventional medical imaging technologies such as MRI (Magnetic Resonance Imaging). Furthermore, FD-OCT has a good signal-to-noise ratio and is fast in measurement.
However, tomographic images generated using FD-OCT are susceptible to fixed horizontal stripe noise due to the DC spectrum of a light source. Thus, there have been researches for reducing such fixed pattern noise.
For example, a reference spectrum subtraction method reduces noise by obtaining the frequency spectrum of an optical power in a physical way and subtracting it from the depth information. However, this method is inconvenient due to the necessity for obtaining the DC spectrum of the light source before a sample scanning.
Some techniques that use software for subtracting fixed stripe noise has been introduced to solve the above inconveniences. Of those techniques, a mean-line subtraction method calculates the mean of the signals and subtracts the mean from each depth information signal. The median-line subtraction method is similar to the mean-line subtraction method except that it uses the median of the signals instead of the mean of the signals.
However, in the above-described conventional methods, if the intensity of the light source changes during measurement, i.e. while the depth information is obtained, noise cannot be reduced normally, which becomes a big problem particularly when an optical fiber is used for optical coherence tomography. By way of example, in the case of medial endoscopy, the intensity of the light source can fluctuate dynamically due to external circumstances such as heartbeats, blood circulation, and breathing. In particular, when an endoscopic tube is bent in a blood vessel, it can cause radical fluctuations in the light source.
Therefore, a need exists for providing an apparatus and a method for efficiently reducing noise, when an unstable light source is used or when the light source is fluctuating dynamically in an optical-fiber-based environment such as an endoscope.
In this regard, Korean Patent Laid-open Publication No. 10-2011-0036593 (entitled “Imaging apparatus and imaging method using optical coherence tomography, and computer readable storing medium”) discloses an apparatus and method for reducing noise caused by a return light.
Also, Korean Patent Laid-open Publication No. 10-2006-0120165 (entitled “Method and apparatus for performing optical imaging using frequency-domain interferometry”) discloses an apparatus and method for reducing shot noise.